1. Field of the Invention
The disclosure relates to a sidelight planar lighting device.
2. Description of the Related Art
Sidelight planar lighting devices (backlights) are widely used as lighting units for liquid-crystal display panels especially in the field of small mobile information apparatuses, such as mobile phones. Sidelight planar lighting devices each include small and environmentally compatible light-emitting diodes (LEDs) disposed along a side end surface of a light guide plate. In such planar lighting devices, the LEDs are generally mounted on a strip-like circuit board and disposed facing a light incident surface of the light guide plate. A front portion of the circuit board is fixed to a portion closer to the light incident surface on the light guide plate with an adhesive member, such as a double-sided tape, whereby the LEDs are disposed and fixed to the light guide plate (refer to Japanese Laid-open Patent Publication No. 2005-259374, for example).
Typically, the arrangement configuration of an adhesive member fixing a circuit board to a light guide plate has a limit in mounting and/or optical characteristics. It may possibly be necessary to dispose small pieces of the adhesive member independently and intermittently (in a manner separated from one another) in some cases. Especially in arrangement of a double-sided tape suitably used as an adhesive member on flexible printed circuits (FPC) suitably used as a circuit board in a planar lighting device, however, it is difficult to reliably and stably fix small pieces of the double-sided tape to the FPC in a manner separated from one another because of the following reasons 1) to 3).
1) The area of each small piece of the double-sided tape (that is, the adhesive area to the FPC) is too small to secure sufficient bonding strength to the FPC. 2) Because the FPC needs to be exposed to high temperature to mount LEDs on the FPC, the process of fixing the double-sided tape to the FPC needs to be performed after the LEDs are mounted on the FPC. This processing order may possibly result in reduced workability in the process of manually fixing the small pieces of the double-sided tape to the FPC, thereby making it difficult to stably fix them at uniform pressure. 3) The double-sided tape may possibly be hard to adhere depending on the material of a cover lay of the FPC or an adherend, such as a light reflection member and a light absorption member, provided on the cover lay.
The double-sided tape generally includes a protection member (hereinafter, referred to as a release liner) that protects adhesive layers on both the front and back surfaces when the double-sided tape is not used. To bond the FPC to the light guide plate with the double-sided tape, the release liner needs to be removed. If it is difficult, because of the reasons described above, to reliably and stably fix the small pieces of the double-sided tape to the FPC in a manner separated from one another, a part of the small pieces of the double-sided tape may possibly peel off the FPC when the release liner is removed after the double-sided tape is fixed onto the FPC. Typically, if at least one of the small pieces of the double-sided tape peels off the FPC, the FPC (that is, the LEDs) fails to be stably fixed to the light guide plate. This leads to reduction in the yield or complication of the process, such as a need for restoration.
To meet a recent demand for planar lighting devices having higher luminance, it is necessary to further increase the number of LEDs disposed along the light incident surface of the light guide plate. Consequently, the LEDs are designed to have a narrower pitch (that is, the gap between adjacent LEDs is made narrower), resulting in an increasing need for the adhesive members having an arrangement configuration separated from one another. As described above, to further increase the luminance of the planar lighting device by further increasing the number of LEDs, it is necessary to use adhesive members disposed in a manner separated from one another. The use of such adhesive members, however, makes it difficult to reliably and stably fix the FPC to the light guide plate.